Planets

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Planets
Exoplanet Kepler-20 Artist's impression
An artist's impression of the exoplanet Kepler-20
Credit: NASA/Ames/JPL-Caltech

A planet is an object which orbits a star.

Planets must have three things to be classed as planets and these are:

  • Be spherical in shape, rounded by its own gravity.
  • Be small enough not to becomes stars (by carrying out nuclear fusion).
  • Have cleared its local area of other objects, such as asteroids.

Objects which are too small to have done these things are called dwarf planets instead.

Planets divide into 2 main groups: terrestrial planets and giant planets. Terrestrial planets have a solid surface, like the Earth. Giant planets are made of layers of gas, or ice, with small rocky cores.

The terrestrial planets in our Solar System are:

The giant planets in our Solar System are:

Planets which orbit other stars (i.e. NOT the Sun) are called exoplanets.

Facts and Figures Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune
a 1 AU (or Astronomical Unit) = 149,600,000 km and is the mean distance from the Earth to the Sun
b 1 Earth Mass = 5,980,000,000,000,000,000,000,000 kg
c 1 g = 9.8 m/s2 - this is the acceleration due to gravity on Earth
d All values are sidereal days (time taken to rotate once) except for Mercury and Venus which are solar days (time from midnight to midnight).
Orbital distance (AU) a 0.38 0.72 1.00 1.52 5.20 9.54 19.20 30.06
Radius (km) 2,440 6,054 6,378 3,397 71,492 60,268 25,559 24,746
Mass (Earth Masses) b 0.055 0.82 1.00 0.11 318 95.2 14.5 17.1
Year Length (Earth days) 88 225 365.25 687 11.9 years 29.45 years 84.0 years 164.8 years
Day Length (Earth Days)d 176 117 1.0 1.03 0.41 0.43 0.75 0.67
Surface Gravity (g) c 0.38 0.91 1.0 0.38 2.34 0.93 0.92 1.12
Surface Temperature (approx.) -200 to 400°C 460°C -80 to 50°C -150 to 20°C -110°C -140°C -190°C -200°C
Number of Moons 0 0 1 2 90+ 140+ 27 14